To better phrase that, if some are safe running 12psi out of large-ish turbos like T3S60s and stuff, am I safe running 15-17psi for short bursts out of a T25 as long as I can keep the intake temps down (using WI)? Are the rings in the Miata motors limited by actual pressure or power?

 

You are really limited by the quality of your tune up to about 270-300hp. That is what it seems like. Remember that there are people that blow up their motors at less boost and less hp.

 

boost indirectly limits how much you can safely run through the motor. Two things cause great stress for the motor given it is tuned right: high hp AND high rpm. The rods are the first to go.

You will be pushing the envelope on stock rods at 275+whp but any increase in power will results in added stress to the motor in any case.

Of course proper cooling, etc have to be present to support the power level you are running.

 

Actually I believe the two would actually be torque and high rpm. Horsepower is a function of torque and rpm whereas torque is a calculation of how much power each stroke of a piston is putting down, which in turn estimates the ammount of force pushing down on the piston (which pushes on the wristpin, which pushes on the rod, which pushes on the crank). High rpm causes a slightly different sort of stress.

 

of course hp is derived from torque but I mentioned hp since it is directly related to torque and is always the higher curve past 5252 RPM.

and high RPM simply caused the moving parts to be accelerated faster and stress is thus applied and removed at a higher rate causing .... well more stress on the metal.

 

you have a good point. I can see how a constant torque would create far greater stress at 7k rpm as opposed to 4k rpm due to the rate at which force is applied and removed. So maybe hp is a better general indicator of the capabilities of a motor given a good tune?

 

I'd venture to guess that 90% of snapped rods in Miatas are from knock. Not many people run race gas in their Miatas, especially non-built ones, but I bet you wouldn't see the same failures at 300whp on race gas as people wouldn't be knocking.

Frank

 

True nock and shock laoding like the ones who where running 270+ whp and put in a wet shot of NOS.

 

well the torque and hp and boost and knock are all going to have an effect on.... cylinder pressures. while power can influence them, it wont have nearly the effect of detonation, preignition, or knock. compressing an explosion too early is not good for rods.

but there are also reports of missed-shift rod failures. which makes me wonder if the high vacuum forces are slightly stretching them and weakening them.

 

high vacuum at high RPM is very detrimental to rod bolts due to stretching - this is why it is recommended to change the rod bolts when doing an engine build.

However, like it was said, a lot of the failures are due to a poor tune or something going wrong. You don't need race gas to run 20psi on the Miata motors, just enough ignition retard and fuel.

 

ignition retard whats that we wanna advance ours

 

unless you're ignition is behind LPP, then output falls off with diminishing returns.


Is this where lowering compression will add power...because now more boost can be added without detonation or pre-ignition?

 

boost effectively raises compression ... it has a similar effect to running a higher static compression ratio. So you have to make a compromise at some point between boost, compression ratio, and ignition timing. I run 7 degrees advance timing at my maximum torque point and boost to be on the safe side. If you really want to run high octane, run some natural gas as fuel Or you can do some work on the combustion chamber to remove hot spots.

But realistically, a 9:1 compression ratio will let you run lots of boost ... easily over 20psi on 94 octane pump gas and well into the 300whp territory. Also throw enough fuel in there to keep combustion fairly cool. The stock rods will go before you've maxed power due to detonation.

 

this thread is making me question my supertech 8.6:1 piston purchase.

 

8.6 compression is not that low - it should be ok on and off boost. A slightly higher compression helps spoolup time and torque off boost while the slightly lower compression will give you a bit more of a safety margin and may allow you to use cheaper gas depending on your tune and amount of boost you run.

 

I went for the forged lowered comp pistons because I track the car, and want the extra safety. I thought it was good insurance, and I want to avoid rebuilds.

 

That's exactly why I run race gas, to not have to retard as much..

**** it, if 93 is 3.65, and race gas is 4.85, I know what I'm doing, drinking one less bottle of gray goose per month.

 

Unnecessary cost. The pistion is unlikely to wear out and require replacing, rings on the other hand ... But really, do you think you'll own this car for 10 more years?

 

ignition retard sucks ***, i know why the WI love it sooo much now.

 

what is your guys' minimum ignition advance under boost?